1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device including a tunnel oxide film, more particularly an oxygen-annealed high-temperature oxide (HTO) film formed by chemical vapor deposition.
2. Description of the Related Art
Tunnel oxide films are used in, for example, nonvolatile memory devices such as flash memories. The use of oxygen annealing to reform the structure of an HTO film (SiO2 film) formed by chemical vapor deposition (CVD), thereby creating a tunnel oxide film, is a known art. Normally, the oxygen annealing process is carried out for fifteen minutes at a temperature of 900xc2x0 C. and a low oxygen level such as 1% O2 (99% N2). The annealing conditions are selected so that the thickness of the HTO film will not increase by more than ten angstroms (10 xc3x85) or one nanometer (1 nm), because a greater increase in film thickness might adversely affect device characteristics.
A tunnel oxide film created in this way, however, permits more electrical current leakage than a normal thermal oxide film obtained by oxygen annealing of the surface of a silicon substrate or a polycrystalline silicon (polysilicon) film. Such current leakage may also adversely affect the device characteristics. FIG. 7 is a graph comparing breakdown voltages of a thermal oxide film and a low-oxygen annealed HTO film, both created as tunnel oxide films. The HTO film was annealed in a dry atmosphere under the conditions stated above (15 minutes, 900xc2x0 C., 1% O2). The data in FIG. 7 show that current is more likely to flow through the low-oxygen annealed HTO film than the thermal oxide film. The reason for this is thought to be that in the low-oxygen annealing process, the HTO film is inadequately annealed, so that unpaired silicon electrons remain, creating an electrically leaky film. If the leaky HTO film is used in a nonvolatile memory device such as a flash memory, the charge retention characteristics of the device are impaired, leading to reduced manufacturing yields and reduced device reliability.
For memory applications, there is a particular need to improve the insulation breakdown resistance (breakdown voltage) of tunnel oxide films obtained by oxygen annealing of an HTO (SiO2) film deposited by CVD on a polysilicon film, but if an oxygen annealing process with a high oxidation capability is used so as to adequately reform the structure of the HTO film, oxidation of the tunnel oxide film proceeds so quickly that the underlying polysilicon film is also oxidized.
For a thermal oxide film formed on the surface of a silicon substrate or a polysilicon film, use of a two-stage oxygen annealing process has been proposed to improve the insulation breakdown resistance and reduce current leakage. In the proposed process, a high-temperature oxygen anneal is followed by a low-temperature oxygen anneal, as taught in Japanese Unexamined Patent Application Publication No. 8-255905, for example. The two-stage oxygen annealing process improves the insulation breakdown resistance of the thermal oxide film.
It would be desirable if a similar improvement could be achieved by adequately reforming a tunnel oxide film formed by an oxygen annealing process carried out on the HTO film, so as to more closely approach the insulation breakdown resistance of a thermal oxide film.
An object of the present invention is to provide a method of fabricating a semiconductor device so as to reduce electric current leakage in a tunnel oxide film formed on a polysilicon film.
The invented method includes forming a first film of silicon nitride or silicon oxynitride on a polysilicon layer, forming a second film of silicon oxide on the first film by CVD, and oxygen-annealing the second film to form a tunnel oxide film.
The presence of the silicon nitride or silicon oxynitride film enables the silicon oxide film to be annealed by an annealing process with a high oxidation capability without oxidizing the polysilicon layer. When a silicon nitride film is used, for example, wet oxygen annealing may be carried out for 5 to 10 minutes at 850xc2x0 C. to 900xc2x0 C. When a silicon oxynitride film is used, dry oxygen annealing may be carried out for 30 to 60 minutes at 850xc2x0 C. to 900xc2x0 C., or wet oxygen annealing may be carried out for 5 to 60 minutes at 850xc2x0 C. to 900xc2x0 C. Compared with the conventional low-oxygen annealing process, these processes reform the silicon oxide film more fully, leaving fewer unpaired silicon electrons, thereby reducing current leakage.
If a silicon oxynitride film is used, it may be formed together with the silicon oxide film in a continuous process.
The semiconductor device formed by the invented process may have, for example, a split floating gate comprising a first floating gate and a second floating gate mutually separated by the first film and the tunnel oxide film.